This invention relates to investigations of rock samples, and more particularly relates to nuclear magnetic resonance (NMR) methods for determining characteristics of subsurface rock, including fluid composition.
NMR has been a common laboratory technique for over forty years and has become an important tool in formation evaluation. General background of NMR well logging can be found, for example, in U.S. Pat. No. 5,023,551 to Kleinberg et al., which is assigned to the same assignee as the present invention and herein incorporated by reference in its entirety.
NMR relies upon the fact that the nuclei of many chemical elements have angular momentum (xe2x80x9cspinxe2x80x9d) and a magnetic moment. In an externally applied static magnetic field, the spins of nuclei align themselves along the direction of the static field. This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field (e.g., an RF pulse) that tips the spins away from the static field direction. The angle through which the spins are tipped is given by xcex8=xcex3B1tp/2, where xcex3 is the gyromagnetic ratio, B1 is the linearly polarized oscillating field strength, and tp is the duration of the pulse. Tipping pulses of ninety and one hundred eighty degrees are most common.
After tipping, two things occur simultaneously. First, the spins precess around the direction of the static field at the Larmor frequency, given by xcfx890=xcex3B0, where B0 is the strength of the static field and xcex3 is the gyromagnetic ratio. For hydrogen nuclei, xcex3/2xcfx80=4258 Hz/Gauss, so, for example, in a static field of 235 Gauss, the hydrogen spins would precess at a frequency of 1 MHz. Second, the spins return to the equilibrium direction according to a decay time, T1, which is known as the spin-lattice relaxation time.
Also associated with the spin of molecular nuclei is a second relaxation time, T2, called the spinxe2x80x94spin relaxation time. At the end of a ninety-degree tipping pulse, all the spins are pointed in a common direction perpendicular, or transverse, to the static field, and they all precess at the Larmor frequency. However, because of small fluctuations in the static field induced by other spins or paramagnetic impurities, the spins precess at slightly different frequencies, and the transverse magnetization dephases with a time constant T2.
A standard technique for measuring T2, both in the laboratory and in well logging, uses an RF pulse sequence known as the CPMG (Carr-Purcell-Meiboom-Gill) sequence. As is well known, after a wait time that precedes each pulse sequence, a ninety degree pulse causes the spins to start precessing. Then, a one hundred eighty degree pulse is applied that keeps the spins in the measurement plane, but causes the spins, which are dephasing in the transverse plane, to reverse direction and to refocus. By repeatedly reversing the spins using a series of one hundred eighty degree pulses, a series of xe2x80x9cspin echoesxe2x80x9d appear. The train of echoes is measured and processed to determine the irreversible dephasing, T2.
In rock formations, such as in a borehole environment, T2 for hydrogen-containing fluids (e.g., water, oil, gas) can have significant contributions due to surface relaxation, bulk relaxation, and diffusion effects, i.e.,                               1                      T            2                          =                              1                          T                              2                ,                surface                                              +                      1                          T                              2                ,                bulk                                              +                                    1                              T                                  2                  ,                  diffusion                                                      .                                              (        1        )            
Each of these contributions provides some information about the rock formation and/or about the fluid in the rock formation. For example, in a wetting phase, the surface relaxation contribution, T2,surface, dominates the distribution of observed distribution of decay times, ƒ(T2). Spins relax predominantly due to collisions with the grain surface, with the collision rate being inversely proportional to the pore size. This means that the observed relaxation time is roughly proportional to the pore size, i.e., 1/T2,suface=xcfx81sS/Vp, where S is the surface area of the pore, Vp is the pore volume, and xcfx812 is the surface relaxivity of the rock, a phenomenological parameter that indicates how relaxing the surface is. Thus, for a wetting phase, the observed ƒ(T2) provides information about pore size distribution. In a nonwetting phase, surface relaxation becomes negligible and bulk relaxation, which is related to viscosity, dominates the observed ƒ(T2). Thus, for a nonwetting phase, the observed ƒ(T2) provides information about viscosity.
In a uniform static magnetic field, each spin will experience the same magnetic field strength regardless of its position within the static field, and diffusion will not contribute to the observed ƒ(T2). In a magnetic field gradient, however, each spin will experience different magnetic field strengths as it diffuses through the static field. The Larmor frequencies of the diffusing spins become time dependent, and the series of one hundred eighty degree pulses cannot refocus the spins completely, leading to an additional decay signal. This additional decay signal is proportional to the diffusion coefficient, D, of the fluid and to the square of the gradient strength, g, and the square of the echo spacing, tE, i.e.,                               1                      T                          2              ,              diffusion                                      =                              1            12                    ⁢                      γ            2                    ⁢                      g            2                    ⁢                                    Dt              E              2                        .                                              (        2        )            
As the diffusion coefficient provides an indication of fluid type, measurement of the diffusion effects on ƒ(T2) can be used as the basis for determining the types of fluids in a rock formation.
Certain NMR measurements of diffusion involve changing the echo spacing, tE, in a standard CPMG sequence, and thus the amount of diffusion the spins undergo between echoes, and then comparing the measured relaxations. FIGS. 1A and 1B generally illustrate this approach. FIG. 1A shows two CPMG sequences with different echo spacings, t1 and t2, where t2 is longer than t1. As the echo spacing increases, the spins diffuse further between echoes, and the measured relaxation times will decrease depending on the diffusion coefficient of the fluid, as given in Equation 2 above. FIG. 1B shows the relaxation distributions, ƒ(T2), for an oil and water determined from the two sets of echoes acquired from the two CPMG sequences illustrated in FIG. 1A. As seen in FIG. 1B, the relaxation distribution with the longer echo spacing, t2, is shifted to lower relaxation times, T2, relative to the relaxation distribution with the shorter echo spacing, t1. The size of the shift is proportional to the size of the diffusion coefficient, as indicated by arrows 1 and 2. The shift of ƒ(T2) for a fluid with a small diffusion coefficient 1, such as heavy oil, is smaller than the shift for a fluid with a larger diffusion coefficient 2, such as water or natural gas.
While such NMR diffusion measurements can be useful, they suffer from a number of drawbacks. For example, for a given acquisition time, the two CPMG sequences will not have the same number of echoes. The CPMG sequence with longer echo spacing will have a fewer number of echoes available, so will suffer from lower signal to noise and lower data quality in general. In addition, relaxation distributions for different fluids often overlap, at least partially, making it difficult to identify shifts of individual relaxation times. In cases where the diffusion coefficients for different fluids are small, the shifts may be difficult to distinguish. Finally, these methods cannot separate out the contributions due to diffusion effects from the surface and bulk relaxation contributions in the observed relaxation distributions. Surface relaxation and diffusion have similar effects on the observed relaxation distributions, so these methods may provide inaccurate information about the fluid and about the rock or earth formation under investigation.
The invention provides in one aspect a method for extracting information about a fluid that may be contained in rock or within a portion of earth formation surrounding a borehole (as used hereinafter, the term xe2x80x9crockxe2x80x9d includes earth, earth formation, and a portion of earth formation). For example, certain embodiments of the invention involve generating a sequence of magnetic field pulses in the fluid. The magnetic field pulse sequence includes an initial magnetic field pulse, a first portion that follows the initial magnetic field pulse, and a second portion that follows the first portion. Magnetic resonance signals are detected using the second portion. The first portion of the sequence is then modified, and the magnetic field pulse sequence generated and magnetic resonance signals detected using the second portion again. The magnetic resonance signals are analyzed from a time relative to the initial magnetic field pulse, and information about the fluid extracted.
A second aspect of the invention provides a logging apparatus for implementing embodiments of the inventive measurements in a borehole environment. One embodiment of such a logging apparatus includes a logging tool that is moveable through a borehole and a processor coupled with the logging tool. The processor is programmed with instructions which, when executed by the processor, cause the logging tool to generate a sequence of magnetic field pulses in a region of investigation of earth formation surrounding the borehole. The magnetic field pulse sequence includes an initial magnetic field pulse, a first portion, and a second portion. The processor causes the logging tool to detect magnetic resonance signals produced from the region of investigation using the second portion of the sequence, and then modify the first portion of the sequence and repeat generating the sequence and detecting magnetic resonance signals. The programmed instructions also cause the processor to analyze magnetic resonance signals from a time relative to the initial magnetic field pulse and extract information about the region of investigation.
Further details, features and embodiments of the invention will become more readily apparent from the detailed description that follows.